Currently, over 250,000 primary total hip arthroplasties (THA) are performed each year in the United States alone. The number of revision surgeries is also increasing. Constant improvements in implant design, materials, and surgery techniques help the patient regain a normal lifestyle ever sooner, together with a more promising outlook on long-term success. One recent technological breakthrough in THA is the Minimal Invasive Surgery (MIS) technique for artificial hip joints, which cuts down hospitalization time significantly. With MIS, patients may start ambulating within 24 hours after surgery.
One important goal of THA is to assure sufficient primary stability, guarantee bone ingrowths, and achieve long-term success. Good primary stability can be reached through press-fit of the implant-bone interface. Conventionally, an optimal relationship requires the right tools along with surgeon's experience. As such, proper training and continuous technical improvement help to meet the increasing demand of fully functioning hip replacements.
Total hip displacement surgeries generally consist of different phases, from pre-operative measurements to exposing the proximal femur and performing the osteotomy of the femoral neck. After opening the femoral canal and rasping to the appropriate size, a trial and then the final implant are inserted into the proximal femur. The femoral stem of the final implant is pressed down the femoral canal by hand until it stops, usually 2 to 3 cm above the neck osteotomy. The implant is then tapped with an impactor until its final position is reached, and ideally the implant stops advancing. The right amount of tapping force has to be applied to gain a good press-fit situation of the implant, while the insertion resistance of the implant may increase as the stem is advanced down the canal.
Obtaining the optimal press-fit under the given conditions and, thus, good primary stability often depends on the surgeon's experience. Implant size, rasping technique, bone constitution, and tapping strength are leading factors influencing the outcome of the hip arthroplasty.
If the femoral stem is inserted outside an ideal press-fit bandwidth, it is either too loose and, therefore, lacks primary stability, or too tight, leading to stress concentration and, in extreme cases, fracture of the femur. Advanced surgery techniques, such as MIS, are subject to constraints like limited access to the femoral neck and reduced visibility and acoustical feedback, which increase the risk for effects of sub-ideal implant insertion. Sub-optimal primary stability and fracture of the femur may necessitate bigger incisions and thus contradict the advantages of MIS. New tools need to be developed to overcome these and other limitations of conventional orthopedic implants and to guarantee continuous high quality THA's.
Conditions of osteoarthritis can be affected by cartilage defects. The transplantation of osteochondral (cartilage-bone) plugs is an alternative approach to treat local, full thickness cartilage defects in young patients. The process involves grafting a plug from the patient or a donor and placing it into a predrilled hole in the patient's body. Plug placement is generally conducted with a metal or plastic punch or hammer. During this process, it is difficult to control the amount of the press fit tolerance and the position of the osteochondral plug in the recipient hole.
Inadequate placement of osteochondral plugs may produce abnormal stress and strain distributions within the cartilage, and thus influence the regeneration of the injured cartilage site and the maintenance of opposing, healthy cartilage surfaces. Further, if too much force is used when pounding in the osteochondral plug, cell viability may be adversely affected and stimulation inhibited. Hence, there is a need for tools to better control osteochondral plug impaction and placement during osteochondral plug transplantation. Further, there is a need for tools which can utilize sensors and a feedback loop to determine the proper amount of force to ensure optimal osteochondral plug transplantation.